The cooling of electronic equipment in a casing is usually obtained by evacuating the calories produced inside the casing to the external environment by means of a flow of forced air penetrating into the casing through ventilation orifices situated in the bottom part serving as nozzles, coming into contact with the components of the electronic equipment and being evacuated in the top part of the casing through ventilation orifices serving as vents. The calorie evacuation capabilities increase with the cooling air flow rate. In the case of the aeronautical standards such as Arinc 600 notably, the diameter and the number of the ventilation orifices are constrained so as to respect a headloss for a standardized air flow rate, which is in turn dependent on the dissipated power. This constraint makes it possible to distribute the flow of pulsed air between the different computers.
When an electronic gear casing is also used as electromagnetic shielding, which is almost always the case for equipment onboard an aircraft, the effectiveness of the electromagnetic shielding imposes a smaller diameter on the ventilation holes, the number of which is then limited by the imposed headloss constraints. These limitations on the diameter and the number of the ventilation holes greatly restrict the capabilities of cooling by natural convection and very often make it necessary to limit the usable temperature range in case of loss of the forced ventilation.
The electronic equipment designed to be onboard aircraft is usually placed in casings that are provided with ventilation orifices of a diameter and number that are inadequate to allow normal cooling by natural convection alone and that are placed on a pulsed air distribution box, a kind of organ windchest, distributing to them a flow of cooling air under pressure meeting precise specifications, for example those given in the Arinc 600 standard relating to the configuration of casings and subracks used in aircraft to house replaceable electronic equipment, also known as “rackable” equipment.
Such an arrangement raises the difficult problem of an even distribution of the available pulsed air for the cooling between casings or disparate equipment that do not have the same cooling requirements and that, in addition, are not necessarily present. It also raises the problem, critical when it comes to safety, of the necessary continuity of certain functions handled by the electronic equipment in the case of loss of the forced cooling air flow.
In case of loss of the forced air flow, the cooling is now provided only by natural convection that is ineffective because of the excessively small cross section that can be used of the ventilation orifices that are limited in diameter and in number: in diameter by the requirements of the electromagnetic shielding and in number by the headloss imposed by the standard, and because of the volume of external air available under the casings reduced to the capacity of the pulsed air distribution box. The temperature of the equipment then increases significantly, which degrades their temperature operating range.
Control of the operating temperature of electronic equipment in the case of loss of the forced air flow is the main limitation encountered when seeking to reduce its bulk and increase its functionalities by increasing the density of the electronic circuitry because both are always accompanied by an increase in the production of calories by the liter.
Now, it is not possible to improve the cooling by natural convection in case of loss of the forced cooling air flow, by providing additional air intake orifices in the bottom parts of the casings, outside the distribution box, because these additional intake orifices contravene compliance with the specified headloss requirement, notably that given in the Arinc 600 standard.